1. Field of the Invention:
The present invention relates to the catalytic (co)dimerization of an alkyl acrylate in the presence of certain palladium complexes, and, more especially, to the catalytic dimerization of alkyl esters of acrylic acid to produce alkyl diesters of dihydromuconic acid.
2. Description of the Prior Art:
Certain palladium derivatives are known to this art to catalyze the dimerization of alkyl acrylates. Thus, bis(benzonitrile)palladium(II) chloride, (C.sub.6 H.sub.5 CN).sub.2 PdCl.sub.2, has been used for the dimerization of methyl acrylate; Barlow et al, J. Organometal. Chem., 21, 215 (1970). Nevertheless, this system is not very efficient because, as reported, a conversion of only 67% for a selectivity to dimers of 93%, 90% of which was to straight-chain dimer, was attained over 23 hours at 133.degree. C. The reaction rate was improved by adding silver tetrafluoroborate, AgBF.sub.4, to the reaction mixture; Oehme et al, Tetrahedron Letters, 4, 343 (1979). This reaction rate was also improved by adding p-benzoquinone, according to Pracejus et al, Z. Chem., 20, 24 (1980).
The efficiency of cationic complexes of palladium having the formula Pd(NCMe).sub.4 (BF.sub.4).sub.2, especially in the presence of anhydrous LiBF.sub.4 is also known to this art, because at 40.degree. C. in 30 hours, a 93% yield of methyl acrylate dimers is attained after distillation, the selectivity to the .DELTA..sup.2 -trans isomer being on the order of 93-96%. See Nugent et al, J. Org. Chem., 48, 5364-5366 (1983).
Moreover, the codimerization of 1,3-butadiene and methyl acrylate to form methyl heptadienoates, catalyzed by a mixture of palladium chloride of the .pi.-allyl type, a phosphine and a silver salt is also known to the art. Compare French Patent No. 2,079,319.
The catalysis of the dimerization of an alkyl acrylate, or of the codimerization of such types of compounds with a conjugated diene compound by means of a cationic allyl complex of palladium, modified by the addition of a phosphine, has also been proposed to the art (French Patent No. 2,524,341), the subject complexes being prepared from the palladium compound and dibenzylidene acetone and an allyloxytris(dimethylamino)phosphonium salt and 1,5-cyclooctadiene.
In capsule summary, all of the foregoing reactions, when using the readily available palladium complexes, such as: EQU PdCl.sub.2 (PhCN).sub.2 or [Pd(C.sub.3 H.sub.5)Cl].sub.2
are typically slow and/or not very selective. Any improvement in the activity and/or selectivity of these reactions is accompanied by an increase in the complexity of the palladium compounds, and, therefore, in the cost of preparing them and by increased difficulties in the use thereof, caused, in particular, by their instability.
Thus, serious need exists for dimerization catalytic systems which are both more efficient and more economical than those heretofore proposed to this art.